Process variable transmitters are used in a variety of industrial applications, and provide an electrical output signal corresponding to a sensed condition signal generated by a process variable (e.g., temperature, pressure, pH, etc.) sensor. The electrical output signal of the sensor is translated into a corresponding measurement value for the particular detected environmental variable type. The corresponding measurement value, in turn, is converted into an output signal that is transmitted to a process controller. The process controller performs some action regarding the received output signal.
Typically, the correspondence between the electrical output generated by the transmitter and the measurement value, that is translated from the sensor's signal, is a well-defined, regular (e.g., linear) relationship governed by the range of measured values. For example, in a transmitter utilizing a 4–20 ma. current loop, a 4 milliamp output corresponds to a lowest value in a particular measured range (e.g., zero degrees Celsius) and a 20 milliamp output corresponds a highest value in a particular measured range (e.g., 100 degrees Celsius). Corresponding output signals for measured values in this range are then calculated based upon the relative positions of the measured values in the range. In the case of a linear output encoding scheme, 50 degrees Celsius (the midpoint in the range of 0 to 100 degrees) would result in an output of 12 ma. (the midpoint between 4 and 20 milliamps). In other cases, non-linear encoding schemes are use (e.g., logarithmic).
The relationship between an actual process variable value (e.g., temperature) and a sensor's output signal (e.g., an electrical current or voltage) is generally a non-linear, non-regular relationship. As a consequence the relationship is initially characterized, during manufacturing of the process variable transmitter, through application of known precise process variable inputs, observing the sensor output, and creating a characterization equation that reduces differences between the actual sensor input (e.g., fluid temperature) and the measured process variable value calculated from the sensor output signal. Characterization also linearizes the relationship between an output value and an input process variable.
Thereafter, a calibration correction equation is applied to the characterized calculated process variable values to further improve the accuracy of calculated process variable values over a particular range of process variable input values. Typical transmitter measurement calibrations are two-point calibrations that utilize accurately applied precise environmental variable values at two reference points to render a linear correction definition comprising an offset and slope. The linear correction definition adjusts for differences between actual process variable values and the characterized calculated values rendered by a applying a factory-established characterization equation. Applying the calibration-based linear correction to a characterized calculated measurement value generally renders a reduced error for calculated measured values within the range between the two calibration points—including a minimized error generally, though not necessarily, at the two calibration points. The two calibration sensor inputs are often chosen, by the party performing the calibration, to correspond to the end points of the intended operating range of the transmitter. In the above example, the two calibration points for a temperature transmitter would be at zero and 100 degrees Celsius—the two endpoints of the desired operating range of the temperature transmitter device.
The calibration process it not only time-consuming, but also resource intensive, as it requires applying physical process variable values very accurately to establish the reference measurements. The reference measurements are used to correct a measured signal value during operation of the process variable transmitter. While one of the measured values will be “zero” in many cases, for which a reference point can be established quite easily even without a precision process variable source (e.g., pressure and differential pressure transmitters), establishing a reference point for a second measured value (the other end of a calibrated range) can be a difficult process. As a consequence, recalibrating a process variable transmitter may be impossible in the field, or at least highly impractical, once the process transmitter is installed.